A unisex name is special because it can signify both a boys’ name and a girls’ name.
If all the letters of the alpahbet (that is 26) were equally found in given names, their frequency would be about 3.8%. However, of all common unisex names in the U.S., only about 0.3% begin with the letter X.... Also, only 0.3% of traditional girls' names, such as Ximena or Xena, begin with the letter X, compared to 0.4% of boys' names, most famously Xander or Xavier. Consequently, the initial letter X is slightly more common in boys' names than in girls' and unisex names, and is also one of the rarest initial letters for given names in general.
We classify names as unisex that are not just officially suitable for both boys and girls, but which have actually been used many times in the English-speaking world for both boys and girls in recent years. Some of the unisex names may have more female or male connotations for individuals, but this is often subjective and goes back to the fact that in your own circle of acquaintances, you might have a boy or man, or a girl or a woman, who has this name.